Application Research Of Discrete Element Method In Metal Powder High Velocity Compaction

Discrete element method (DEM) is an effective numerical approach in simulating force behavior of non-continuous materials. It has established itself as an important simulation technique for engineering applications involving granular and large-deformable systems. High velocity compaction (HVC) is a new technology for fabricating high density powder metallurgy parts with low cost and high production capacity. Its productions permit good characters such as high green density, uniform density distribution, low ejection force and low spring-back etc. In the paper DEM is applied for studying the particle flowing process of dense granular system in High Velocity Compaction. According to the characteristics of metal powder materials and the relevant principles of HVC, the numerical model was established based on mechanics of granular media and theory of elastic-plasticity. The densification behavior, stress wave propagation and heat transfer rule of powder system in the compaction process of HVC were analyzed. The main works are concluded as follows:Firstly, we summarized the research situation and application of DEM and the principle and characteristic of HVC. Since the distributing features of raw packing material obey to Gaussian distribution, a new specimen generation method based on Box Muller transform was presented. Considering powder presents successively to work Hardening, work softening and plastic deformation in HVC process, a contact force model was deduced to describe the high strain rate and elastic-perfectly plastic of powder material. The element search method, boundary condition and governing equation of particle flow for HVC were also proposed.Secondly, with the computing program developed based on software Particle Flow Code (PFC), the three-dimensional forming process and density distribution of samples under HVC conditions were simulated. The quantitative capabilities of DEM are provided by comparing numerical models to real experimental data. Numerical results show that the rearranging and porosity filling behavior of metal powder at the early compaction stage have significant influences on green density increasing, at the late stage the densification of particles is mainly in the manner of elastic-plastic deformation. The contrast experiments were performed for different packing fractions, diameter high ratio of compacts and different loading conditions. The simulation results coincide well with experiments in engineering application.Thirdly, evolution features of compaction force in HVC process was simulated based on the DEM. Because the full process was divided into elastic loading, plastic deformation and elastic unloading, the governing equations were established in three stages respectively. The impacting forces through different layers of powders were obtained, which could not be observed by experiments. The simulation results show obviously delay phenomenon and serrate wave with different gradient at loading and unloading processes. The simulated low-level stress waves are compared to available experimental data, which are consistent with the overall waveform.Finally, DEM was applied for studying the high temperature spread in a compacted metal particles system. Assuming that thermal transmissions existed only at the contact zones between particles, the general thermal equation based on continuum mechanics was rewritten to a discrete form. Then the interior heat source generated during friction of the motion particles was deduced from kinetic equations. Using the proposed model, the factor which affect the temperature distribution of specimen was investigated including particles arrangement, particle diameter distribution and green density. Temperature profiles at different locations and different moments are given. At a fixed location the temperature rise presents a logarithmic relationship with time. Simulation results show that the geometry distribution of particle materials significantly influences the temperature distribution. Heat transfers fast in the system which is comprised of small and uniform particles.